1. Field of the Invention
The present invention relates to an RFID interrogator that transmits and receives a signal to and from a wireless tag through wireless communications, and to a data communication method with the wireless tag.
2. Description of the Related Art
There is an RFID (Radio Frequency Identification) technology as a technology for transferring and receiving information to and from the IC tag storing identification information etc. through the wireless communications.
The system employing this RFID technology is configured by an interrogator and a transponder. The interrogator accesses an IC chip defined as the transponder, and reads the information (e.g., ID information) written to this IC chip. Further, the interrogator often writes the information to this IC chip. Moreover, the transponder is the IC chip to which a unique piece of ID information is written, and is given a variety of nomenclatures such as an IC tag, a wireless IC tag, an RFID tag, an electronic tag and a wireless tag. In the following discussion, the transponder shall be called a [wireless tag].
Thus, in the system using the RFID technology, the RFID interrogator (which will hereinafter be simply referred to as an interrogator) reads and writes, in a non-contact state, the information from within and to the wireless tag. The wireless tag is exemplified such as operating by using radio waves transmitted from the interrogator as an energy source and responding in a way that carries the self-stored information on reflected waves of the radio waves.
FIG. 7 illustrates an example of an architecture of this type of conventional RFID system. FIG. 7 shows the example of the architecture in which two interrogators 1001 and 1002 are installed adjacently. Thus, the plurality of interrogators are adjacently installed and respectively perform communications through on the radio waves with the wireless tags, in which case the radio waves transmitted by the respective interrogators interfere with each other between these individual interrogators. For example, the interference of the radio waves transmitted from the interrogator 1002 occurs in the interrogator 1001 in FIG. 7 (interrogator-to-interrogator interference). Further, the respective wireless tags, because of simultaneously receiving the radio waves from the plurality of adjacent interrogators, have the occurrence of the radio wave interference (interrogator-to-tag interference).
Given as a method of avoiding the aforementioned interrogator-to-interrogator interference is a method by which the individual interrogators conduct carrier sense (measurement of interference level in a selected channel) before starting the communications and thus search for an idle channel (refer to Non-Patent document 1). Namely, this method is a method of conducting control so that each interrogator performs the communications by use of a frequency channel different from a channel employed by the other interrogator.
In the system that executes the carrier sense, further, a maximum consecutive transmission period and a minimum transmission stop period are specified in order to impartially employ one channel between the respective interrogators (refer to Non-Patent document 1). FIG. 8 is a diagram showing a relationship between a transmission period (the maximum consecutive transmission period and the minimum transmission stop period) and the carrier sense on the same channel. As illustrated in FIG. 8, the interrogator performing the communications by acquiring a predetermined channel through the carrier sense, has a necessity of stopping the communications during a predetermined transmission stop period as the maximum consecutive transmission period elapses. With this contrivance, the interrogator standing by for the transmission gains a chance for acquiring the channel during the transmission stop period of the interrogator that has been in the process of the previous communications.
A transmission process of the interrogator executing the carrier sense and also transmission control by using the maximum consecutive transmission period and the minimum transmission stop period, will hereinafter be explained with reference to FIG. 9. FIG. 9 is a flowchart showing a transmission processing flow of the interrogator.
The interrogator, on the occasion of communicating with the wireless tag, at first conducts the carrier sense (S1101 through S1107). When conducting the carrier sense, the interrogator, to begin with, sets an initial channel (a predetermined frequency) (S1101). Subsequently, the interrogator measures receiving electric power of the radio waves on this set channel (S1102). If the receiving electric power of the radio waves on the set channel is larger than a threshold value (S1103; NO), it is judged that this channel is already used by the other interrogator, and therefore the interrogator changes the previously-set channel to another channel (S1104).
While on the other hand, when judging that the receiving electric power of the radio waves on the set channel is equal to or smaller than the threshold value (S1103; YES), the interrogator sets a back-off period (a random wait period) (S1105). Then, the interrogator measures the receiving electric power of the radio waves received within the set back-off period (S1106). These processes are control for preventing the interference between the interrogators each performing the carrier sense at the same timing.
When judging that the receiving electric power thereof within the set back-off period is larger than the threshold value (S1107; NO), the interrogator judges that the channel was used by other interrogators during the back-off period, and therefore changes the previously-set channel to another channel (S1104). While on the other hand, when judging the receiving electric power within the set back-off period is equal to or smaller than the threshold value (S1107; YES), the interrogator judges that there does not exist the other interrogator having selected the same channel, and therefore starts the transmission on the set channel.
The interrogator acquiring the predetermined channel through the carrier sense performs the transmission by use of the acquired channel, however, a consecutively transmissible period is within the period specified as the maximum consecutive transmission period (S1108). Upon an elapse of the maximum consecutive transmission period, the interrogator keeps stopping the transmission for a minimum transmission stop period or longer (S1109). If there is a necessity of continuing a further transmission, upon an elapse of the minimum transmission stop period, the interrogator initiates the carrier sense again (S1101 through S1107).
The above-mentioned Non-Patent document 1 is “Electromagnetic compatibility and Radio spectrum Matters (ERM); Radio Frequency Identification Equipment operating in the band 865 MHz to 868 MHz with power levels up to 2 W; Part 1: Technical requirements and methods of measurement” (European Telecommunications Standards Institute, ETSI EN 302 208-1, 2004-09, V1.1.1, p. 10, 26).
However, the carrier sense of the interrogator described above is capable of avoiding the interrogator-to-interrogator interference by changing the channel to be used for each of the interrogators but is incapable of avoiding the interrogator-to-tag interference. This is because the majority of wireless tags do not have a function of a frequency filter etc., so that the data can be normally neither demodulated nor decoded in the case of simultaneously receiving the signals via different frequency channels.
A centralized control method using a control device is effective in reducing the interrogator-to-tag interference so that the transmissions of the plurality of neighboring interrogators do not overlap each other in time. In this method, however, a control procedure on the control device must be determined based on the interference condition among the interrogators, and hence a problem arises, wherein a complicated adjustment of the control procedure in accordance with a interrogator installation environment is required, and the operation becomes complicated.